U.S. patent application number 09/892313 was filed with the patent office on 2001-11-22 for miniature micromachined quadrupole mass spectrometer array and method of making the same.
This patent application is currently assigned to California Institute of Technology, a California corporation. Invention is credited to Chutjian, Ara, Fuerstenau, Stephen D., Orient, Otto J., Rice, John T., Yee, Karl Y..
Application Number | 20010042826 09/892313 |
Document ID | / |
Family ID | 27367361 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010042826 |
Kind Code |
A1 |
Chutjian, Ara ; et
al. |
November 22, 2001 |
Miniature micromachined quadrupole mass spectrometer array and
method of making the same
Abstract
The present invention provides a quadrupole mass spectrometer
and an ion filter, or pole array, for use in the quadrupole mass
spectrometer. The ion filter includes a thin patterned layer
including a two-dimensional array of poles forming one or more
quadrupoles. The patterned layer design permits the use of very
short poles and with a very dense spacing of the poles, so that the
ion filter may be made very small. Also provided is a method for
making the ion filter and the quadrupole mass spectrometer. The
method involves forming the patterned layer of the ion filter in
such a way that as the poles of the patterned layer are formed,
they have the relative positioning and alignment for use in a final
quadrupole mass spectrometer device.
Inventors: |
Chutjian, Ara; (La
Crescenta, CA) ; Fuerstenau, Stephen D.; (Montrose,
CA) ; Orient, Otto J.; (Glendale, CA) ; Yee,
Karl Y.; (Pasadena, CA) ; Rice, John T.;
(Pasadena, CA) |
Correspondence
Address: |
SCOTT C. HARRIS
Fish & Richardson P.C.
Suite 500
4350 La Jolla Village Drive
San Diego
CA
92122
US
|
Assignee: |
California Institute of Technology,
a California corporation
|
Family ID: |
27367361 |
Appl. No.: |
09/892313 |
Filed: |
June 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09892313 |
Jun 26, 2001 |
|
|
|
09499708 |
Feb 8, 2000 |
|
|
|
Current U.S.
Class: |
250/283 |
Current CPC
Class: |
Y10S 977/773 20130101;
H01J 49/4215 20130101; Y10S 977/891 20130101; H01J 49/0018
20130101; H01J 49/009 20130101 |
Class at
Publication: |
250/283 |
International
Class: |
H01J 049/00; B01D
059/44 |
Goverment Interests
[0002] The invention described herein was made in the performance
of work under a NASA contract and is subject to the provisions of
Public Law 96-517 (35 U.S.C. 202) in which the Contractor has
elected to retain title.
Claims
What is claimed is:
1. A method of mass-filtering an ion beam, the method comprising:
(a) receiving the ion beam through an entrance device with at least
one entrance aperture; (b) providing at least a pair of facing
micromachined conducting rods with an entrance side and an exit
side, the entrance side of the conducting rods adjacent the
entrance apertures; (c) exiting said ion beam as mass-filtered ions
through an exit device located at a distal end from the entrance
device and adjacent the exit side of the conducting rods and having
at least one exit aperture; (d) spacing the conducting rods from
the entrance device and spacing the conducting rods from the exit
device using spacers; and (e) receiving the mass-filtered ions by a
detector.
2. The invention as set forth in claim 1, wherein: the entrance
device is a plate with a concave surface for receiving the ion
beam; and the exit device is a plate with a concave surface for
exiting the ion beam.
3. The invention as set forth in claim 1, wherein the entrance and
the exit devices are gold plates comprised of a silicon substrate
coated with a gold/chromium film outer layer.
4. The invention as set forth in claim 1, wherein the entrance and
the exit devices are titanium plates.
5. The invention as set forth in claim 1, wherein the conducting
rods are non-magnetic, metallic poles.
6. The invention as set forth in claim 5, wherein the non-magnetic,
metallic poles are at least one of: (a) gold; and (b) titanium.
7. The invention as set forth in claim 1, wherein the conducting
rods have a hyperbolic shape defined by an original
Mathieu-equation quadrupole formulation.
8. The invention as set forth in claim 1, wherein the conducting
rods have a cylindrical shape.
9. The invention as set forth in claim 1, wherein the conducting
rods have any shape suitable with negligible loss in mass
resolution.
10. The invention as set forth in claim 1, wherein the conducting
rods are configured with appropriate shapes and lengths such that
they operate at suitably low RF frequencies.
11. The invention as set forth in claim 10, wherein the lengths of
the conducting rods are in a range sufficient to allow operation at
frequencies less than 50 MHz.
12. The invention as set forth in claim 10, wherein the length of
the conducting rods is at least 3 millimeters.
13. The invention as set forth in claim 1, wherein the spacers are
diffusion-bonded to the conducting rods and the en trance and the
exit apertures.
14. The invention as set forth in claim 13, wherein the spacers are
anodically bonded.
15. The invention as set forth in claim 1, wherein the spacers are
made of an insulating material.
16. The invention as set forth in claim 7, wherein the spacers are
glass.
17. A miniature quadrupole mass spectrometer array for analyzing an
ion beam, comprising: a plurality of micromachined entrance
apertures for receiving the ion beam and a plurality of
micromachined exit apertures located at a distal end from the
entrance apertures and for providing said ion beam with egress as
mass-filtered ions; a first set of micromachined conducting rods
and a second set of micromachined conducting rods facing the first
set of conducting rods, wherein both of the first and second
conducting rods are adjacent and between the entrance and exit
apertures; and a first set of micromachined spacers located between
the first and second set of conducting rods and the entrance
aperture and a second set of micromachined spacers located between
the first and second set of conducting rods and the exit aperture,
wherein the micromachined apertures, conducting rods, and spacers
form a miniature micromachined array; and a detector located
adjacent the exit aperture for receiving the mass-filtered
ions.
18. The invention as set forth in claim 17, wherein the array of
devices operate in parallel.
19. The invention as set forth in claim 17, wherein the entrance
device is a plate with a concave surface for receiving the ion beam
and the exit device is a plate with a concave surface for exiting
the ion beam.
20. The invention as set forth in claim 17, wherein the entrance
and exit devices are gold plates comprised of a silicon substrate
coated with a gold/chromium film outer layer.
21. The invention as set forth in claim 17, wherein the conducting
rods have any shape suitable with negligible loss in mass
resolution.
22. The invention as set forth in claim 17, wherein in the
conducting rods are configured with appropriate shapes and lengths
such that they operate at suitably low RF frequencies.
23. The invention as set forth in claim 17, wherein the spacers are
at least one of diffusion-bonded and anodically bonded to the
conducting rods and the entrance and exit devices.
24. The invention as set forth in claim 17, further comprising a
plurality of bonding pads and a plurality of connecting strips,
wherein each of the connecting strips is located between a
respective bonding pad and one of the conducting rod, wherein each
of the bonding pads provides additional structural strength, and a
site for wire bonding to provide a secondary method of electrical
connectivity.
25. The invention as set forth in claim 24, wherein the bonding
pads have an alternate positive and negative pole arrangement.
26. The invention as set forth in claim 25, wherein the alternate
pole arrangement is defined by an outer conductive track and an
inner conductive track, wherein the tracks provide parallel access
to positive and negative poles, respectively.
27. The invention as set forth in claim 17, wherein the array
consists of a range 10 to 10,000 conducting rods, apertures, and
spacers defending on the desired results.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority under 35
U.S.C. .sctn. 119 to U.S. Provisional patent application No.
60/048,540, filed Jun. 3, 1997. The entire contents of U.S.
Provisional patent application No. 60/048,540 are incorporate
herein, as if set forth herein in full.
FIELD OF THE INVENTION
[0003] The present invention generally relates to quadrupole mass
spectrometers. In particular, the present invention relates to a
miniature micromachined ion filter for use in a quadrupole mass
spectrometer, a quadrupole mass spectrometer including the ion
filter, and methods of making the ion filter and the quadrupole
mass spectrometer.
BACKGROUND OF THE INVENTION
[0004] Mass spectrometers are workhorse instruments finding
applications in many commercial and military markets, with
potential for use in domestic markets as well. A mass spectrometer
is able to sample, in situ, the atmosphere in which it is placed
and provide a reading of the atomic and molecular species (and any
positive or negative ions) present in that atmosphere and of the
absolute abundance of these species.
[0005] There are many types of mass spectrometers, such as magnetic
sector, Paul or Penning ion trap, trochoidal monochromator, and the
like. One popular type of mass spectrometer is the quadrupole mass
spectrometer (QMS), first proposed by W. Paul (1958). In general,
the QMS separates ions with different masses by applying a direct
current voltage and a radio frequency ("RF") voltage on four rods
having hyperbolic or circular cross sections and an axis
equidistant from each rod. Opposite rods have identical potentials.
The electric potential in the quadrupole is a quadratic function of
the coordinates.
[0006] Ions are introduced in a longitudinal direction through a
circular entrance aperture located at the ends of the rods and
centered on the midpoints between rods. Ions are deflected by the
field depending on their atomic mass-to-charge (m/z) ratio. By
selecting the applied voltage amplitude and frequency of the RF
signal, only ions of a selected m/z ratio exit the QMS along the
axis of a quadrupole at the opposite end and are detected. Ions
having other m/z ratios either impact the rods and are neutralized
or deflect away from the centerline axis of the quadrupoles.
[0007] As explained in Boumsellek, et al. (1993), a solution of
Mathieu's differential equations of motion in the case of round
rods provides that to select ions with a m/z ratio using an RF
signal of frequency f and rods separated by a contained circle of
radius distance R.sub.0 the peak RF voltage V.sub.0 and DC voltage
U.sub.0 should be as follows:
V.sub.0=7.233 mf.sup.2R.sup.2.sub.0
U.sub.0=1.213 mf.sup.2R.sup.2.sub.0
[0008] Conventional QMS's weigh several kilograms, have volumes of
the order of 10.sup.4 cm.sup.3, and require 50-100 watts of power.
Further, these devices usually operate at vacua in the range of
10.sup.-6-10.sup.-8 torr in order that the mean free path be
comparable to the instrument dimensions, and where secondary
ion-molecule collisions cannot occur. Commercial QMS's of this
design have been used for characterizing trace components in the
atmosphere (environmental monitoring), automobile exhausts,
chemical-vapor deposition, plasma processing, and
explosives/controlled-substances detection (forensic applications).
However, such conventional QMS's are not suitable for spacecraft
life-support systems and certain national defense missions where
they have the disadvantages of relatively large mass, volume, and
power requirements. A small, low-power QMS would find a myriad of
applications in factory air-quality monitoring, pollution detection
in homes and cars, protection of military sites, and protection of
public buildings and transportation systems (e.g., airports,
subways, and harbors) against terrorist activities.
[0009] One type of miniature QMS (U.S. Pat. No. 5,401,962) was
developed by Ferran Scientific, Inc., San Diego, Calif. and
includes a miniature array of sixteen rods comprising nine
individual quadrupoles. The rods are supported only at the detector
end of the QMS by means of powdered glass that is heated and cooled
to form a solid support structure. The electric potential and RF
voltage are applied by the use of springs contacting the rods. The
Ferran QMS dimensions are approximately 2 cm diameter by 5 cm long,
including a gas ionizer and detector, and has an estimated mass of
50 grams. The reduced size of the Ferran QMS results in several
advantages over existing QMS's, including a reduced power
consumption and a higher operating pressure.
[0010] The Ferran QMS has a resolution of approximately 1.5 amu in
the mass range 1-95 amu. This is a relatively low resolution for a
QMS, making the miniature Ferran QMS useful for commercial
processing (e.g., chemical-vapor deposition, blood-plasma
monitoring) but not for applications that require accurate mass
separation, such as in analytical chemistry and in spacecraft
life-support systems. Boumsellek et al. (1993) traced the low
resolution to the fact that the rods were aligned only to within a
.+-.3% accuracy, whereas an alignment accuracy in the range of
+0.1% is necessary for a high resolution QMS.
[0011] A separate miniature QMS (U.S. Pat. Nos. 5,596,193 and
5,719,393) was developed by the Jet Propulsion Laboratory (JPL),
California Institute of Technology to address the continuing need
for a reduced size QMS having an acceptable rod alignment. The JPL
QMS provides improved resolution over the Ferran QMS due to
improved accuracy in rod alignment. As may be appreciated, the
accurate positioning and alignment of individual miniature rods in
an array significantly increases the cost of manufacturing due to
the increased time and specialized equipment required for precisely
aligning separate miniature rods. As the size of the rods is
further reduced, the complexity, difficulty and expense of rod
positioning and alignment increases. In this regard, there is a
need for a small QMS having high resolution that may be made by
simpler and less expensive manufacturing process.
SUMMARY OF THE INVENTION
[0012] In one aspect, the present invention provides a quadrupole
ion filter, and a quadrupole mass spectrometer including the ion
filter, that avoids problems associated with miniaturization of
conventional quadrupole mass spectrometer devices, and especially
problems concerning the incorporation of loose rods into
conventional devices. The ion filter includes a patterned layer of
electrically conductive material, with the patterned layer
including a two-dimensional array of poles for one or more
quadrupoles. Alternatively, the ion filter may be described as a
pole array. The pole array, or array of poles, in the pattern is
two-dimensional in that the poles in the array have a regular
spacing in the x-y plane, with the length of the poles in the array
being in the z direction. The poles of the ion filter serve the
same function as the rods in conventional quadrupole devices. The
patterned layer is divided into a number of separate sections, or
pieces, each including at one terminal end one pole in the array of
poles. At the other terminal end of each separate piece is a
bonding location for convenient electrical connection of the piece
with an external power source.
[0013] Structurally, the quadrupole ion filter of the present
invention is considerably different than the quadrupole structure
in conventional quadrupole mass spectrometers. Conventional
quadrupole mass spectrometers, even those that have been
miniaturized, use poles that are in the form of individual
longitudinally extending rods. The ion filter of the present
invention, however, includes the array of poles in a thin patterned
layer, with the thickness of the layer corresponding with the
length of the poles.
[0014] The patterned layer in the ion filter of the present
invention typically has a thickness of smaller than about 6
millimeters, although even smaller thicknesses may be preferred for
some applications. In that regard, the thinner that the patterned
layer is, the shorter the length of poles and, therefore, the
shorter the distance that ions must travel to pass through the ion
filter. A shorter length of travel through the ion filter permits
operation at higher pressures, which is a significant advantage
with the ion filter of the present invention.
[0015] By use of the patterned layer in the ion filter of the
present invention, it is possible to make the poles of an extremely
small size and with an extremely dense spacing. For example, with
the present invention, the density of poles in the patterned layer
is typically greater than about 2 poles per square millimeter, and
in many embodiments the density is much higher. Furthermore,
directly opposing poles in the patterned layer are typically
separated by a distance of shorter than about 0.2 millimeter, and
in many embodiments by an even shorter distance. Diagonally
opposing poles in the patterned layer are typically separated by a
distance of shorter than about 0.3 millimeter, and in many
embodiments by an even shorter distance. Because of the extremely
small size and dense spacing of the poles, the ion filter may
include a large array of poles in a small space, with different
groupings of four adjacent poles each defining a channel for
passage of ions. With the present invention, however, these
quadrupole channels are extremely small. When the ion filter
includes a large array of poles, defining a plurality of quadrupole
channels, the channels are typically present in a density of larger
than about one of the quadrupole channels per square millimeter,
and often greater than two of the quadrupole channels per square
millimeter.
[0016] An advantageous structure for the ion filter of the present
invention is one in which substantially all of the patterned layer
is supported by a single, common supporting substrate, which is
typically of dielectric material. The patterned layer is such,
however, that a portion of the patterned layer that includes the
poles is suspended from the substrate. Typically, the suspended
portion of the patterned layer extends over an opening that passes
through the substrate. In this way, the opening provides a
passageway to permit ions access to the quadrupole channels. The
patterned layer is bonded to the supporting substrate in a manner
that maintains positioning and alignment of the poles, even though
the poles are suspended from the substrate.
[0017] A significant aspect of the present invention is manufacture
of the quadrupole ion filter, and manufacture of quadrupole mass
spectrometers including the ion filter. According to the present
invention, a method is provided in which the poles in the patterned
layer are made in a manner such that as the poles are made they
have relative positioning and alignment for final use in a
quadrupole mass spectrometer. This is typically accomplished,
according to the method of the present invention, by forming the
patterned layer of the ion filter on a common supporting substrate
so that the patterned layer, as formed on the common supporting
substrate, is bound to the substrate, such that the relative
positioning and alignment of poles in the patterned layer is
thereby fixed.
[0018] One preferred embodiment of the method for manufacturing the
ion filter involves simultaneous manufacture of the patterned
layer, including the poles, by filling a mold with electrically
conductive material. The mold includes a template for the patterned
layer. The mold is filled when it is situated on the surface of the
common supporting substrate. When the mold is then removed, the
patterned layer remains supported by the common supporting
substrate. In one embodiment, the mold may be made by a technique
known as Lithographie-Galvanoformung-Abformung (LIGA)
manufacture.
[0019] Another embodiment of the method for manufacturing the
present invention involves forming the patterned layer from a
single work piece, typically in the form of a metallic sheet, that
has been bonded to the common supporting substrate. Material is
selectively removed from the work piece to form the patterned
layer, such that the patterned layer, as formed, is bound to and
supported by the common supporting substrate. Typically, the
selective removal of material from the work piece is accomplished
by electrical discharge machining (EDM).
[0020] The present invention also involves a quadrupole mass
spectrometer including the mass filter of the present invention.
The quadrupole mass spectrometer includes the ion filter located
between an ion source and an ion detector. During operation, the
ion source supplies ions to be filtered by the ion filter. Ions
passing through the ion filter may then be detected by the ion
detector. The quadrupole mass spectrometer may include spacers
before and/or after the ion filter to maintain a predetermined
spacing between the ion filter and the ion source and/or the ion
detector and to assist in isolating the operation of the ion filter
from influences from other components. These spacers are typically
made of dielectric material. The quadrupole mass spectrometer may
also include entrance and/or exit devices for enhancing performance
of the quadrupole mass spectrometer. The entrance device is located
between the ion source and the ion filter and typically-includes a
body of dielectric material having apertures therethrough for
channeling ions from the ion source into the ion filter. In a
preferred embodiment, the entrance device includes an electrically
conductive metallic film at least on a side facing the ion source,
to dissipate the charge of ions striking the entrance device. The
exit device similarly includes a body of dielectric material having
apertures therethrough for channeling ions exiting the mass filter
to the ion detector. In a preferred embodiment, the exit device
includes an electrically conductive metallic film on at least a
side facing the ion filter, to dissipate the charge of ions
striking the exit device.
[0021] Furthermore, the quadrupole mass spectrometer has a
versatile design that may be adapted to a variety of situations.
For example, a Faraday-type ion detector may be used for operation
at relatively high pressures, often in the millitorr range. For
operation of the device at very low pressures, such as those below
about 10.sup.-4 torr, a single particle multiplier may be used as
the ion detector.
[0022] Also, according to the present invention, the quadrupole
mass spectrometer including the ion filter may easily be
manufactured through proper alignment and assemblage of the
individual components.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a block diagram showing major components of one
embodiment of a quadrupole mass spectrometer of the present
invention;
[0024] FIG. 2 is a partial top view, drawn to a large scale, of one
embodiment of an array of poles in an ion filter of the present
invention.
[0025] FIG. 3 is a perspective view of one embodiment of an ion
filter of the present invention;
[0026] FIG. 4 is an exploded view in perspective illustrating
several of the components and their arrangement in one embodiment
of a quadrupole mass spectrometer of the present invention;
[0027] FIG. 5 is a partial cross section through a single pair of
metallic poles of one embodiment of a quadrupole mass spectrometer
array of the present invention;
[0028] FIG. 6 is a partial perspective view of a bonding pad
configuration with connecting strips attached to alternate poles of
one embodiment of a quadrupole mass spectrometer of the present
invention;
[0029] FIG. 7 is a top view of one embodiment of a bonding
configuration for making electrical connection to poles of in ion
filter of the present invention;
[0030] FIG. 8 is a flow diagram illustrating one embodiment of a
LIGA-based process of the present invention for making an ion
filter for use in a quadrupole mass spectrometer;
[0031] FIG. 9 is a flow diagram illustrating one embodiment of an
EDM-based process of the present invention for making an ion filter
for use in a quadrupole mass spectrometer.
DETAILED DESCRIPTION
[0032] The present invention provides a quadrupole mass
spectrometer comprising an ion source, an ion filter, and an ion
detector, useful for in situ sampling of an atmosphere for
identification of atomic and molecular species that may be present
in the atmosphere. The present invention also includes an ion
filter for use in the quadrupole mass spectrometer including an
array of at least 4 miniature poles defining at least one
quadrupole channel through which ions pass for detection. This ion
filter can may also be described as a pole array. The pole array,
or array of poles is typically used to perform the ion filtering
function in the mass filter component of the quadrupole mass
spectrometer. The ion filter typically comprises a sufficiently
large two-dimensional array of poles to define a plurality of
quadrupole channels in a quadrupole mass spectrometer array (QMSA).
Having a plurality of quadrupole channels is advantageous to
enhance detection sensitivity, especially for the miniature device
of the present invention because the detection sensitivity
associated with a quadrupole channel generally decreases with
decreasing channel size, due to the smaller cross-sectional area of
the channel that is available for passage of ions.
[0033] Referring now to FIG. 1 the major components of the
quadrupole mass spectrometer of the present invention are shown. As
illustrated in FIG. 1, a miniature micromachined quadrupole mass
spectrometer 10 is shown including an ion source 28, an ion filter
29, and an ion detector 32. The mass spectrometer 10 operates
according to known principles. During operation, the ion source 28
provides ions in an ion beam 22. Ions in the ion beam 22 travel to
the ion filter 29 where ions are filtered according to the m/z
ratio of the ions, with m referring to the mass of an ion and z
referring to the charge of an ion. Mass filtered ions 31 exiting
the ion filter 29 may then be detected by the ion detector 32. At
any given time, the mass filtered ions 31 include substantially
only ions in a narrow range of m/z ratios, so that the ion detector
32, at any given time, is detecting only ions within the narrow
range. The location of the m/z range of the mass filtered ions 31
may be periodically or continuously varied by varying RF frequency
and voltages to the ion filter 29, as discussed further below,
using control electronics known in the art. In this way, the mass
spectrometer may be used to detect ions over a wide range of m/z
values. Information from the ion detector 32 concerning detected
ions may be interpreted by techniques known in the art for
identification of atomic and molecular species originally present
in the atmosphere being sampled by the mass spectrometer 10.
[0034] The ion source 28 may be any apparatus capable of generating
ions for filtering in the ion filter 29. Examples of the ion source
28 include a field-emission ionizer and an electron-impact ionizer.
Preferred as the ion source 28 is an electron-impact ionizer.
[0035] The ion detector may be any apparatus capable of detecting
the mass filtered ions 31. Examples of the ion detector 32 include
a Faraday-type ion detector, a single-particle multiplier and a
flat micromachined plate. Preferred as the ion detector 32 is a
miniature micromachined-plate ion multiplier.
[0036] The ion filter 29 includes the QMSA of the present invention
as an active element for filtering ions for detection. The QMSA
filters ions based on general principles well known in the
operation of quadrupole mass spectrometers. The QMSA of the present
invention, however, can be of an extremely small size, which is
advantageous for many uses, especially when size or weight
considerations are important, such as in space applications. Also,
the QMSA of the present invention is manufacturable by
micromachining techniques that lend themselves to relatively high
volume, low cost manufacture.
[0037] One embodiment of the QMSA of the present invention is shown
in FIG. 2, including an array of poles 16, with any grouping of
four adjacent poles 16 defining a quadrupole channel 17 through
which ions travel during use. The quadrupole channel 17 refers to
the space defined by any grouping of four poles 16 within areal
boundaries defined by a circle that is substantially tangent to
each of the four relevant poles 16, as exemplified by the dotted
circles shown for two of the quadrupole channels 17 in FIG. 2. Each
of the poles 16 form an integral structure with a connecting strip
50, which acts as an electrical lead to the respective one of the
poles 16. Each of the poles 16, therefore, forms the terminal
portion of an integral piece including one of the poles 16 and a
corresponding connecting strip 50.
[0038] With continued reference to FIG. 2, each of the poles 16 has
either one or two curved exterior surfaces 19, such that each of
the quadrupole channels 17 has four of the curved surfaces 19
facing the quadrupole channel 17. The curved surfaces 19 as shown
in FIG. 2 have a hyperbolic shape, which is preferred for the poles
16. Other surface shapes, could, however, be used, such as an arc
of a circle.
[0039] In a conventional quadrupole mass spectrometer, the poles
would be separate pieces, such as individual circular rods,
assembled in an array. With reference to FIG. 2, the poles 16 of
the QMSA of the present invention are significantly different than
the poles in conventional quadrupole mass spectrometers, because
the poles 16 are a terminal portion of a larger integral structure,
as noted above. The terminal portions forming the poles 16 of the
present invention generally include only the terminal portions of
the integral structure generally within the area defined by the
curved surfaces 19, as shown by the dotted lines shown for two of
the poles 16 in FIG. 2. One significant advantage of the poles 16
of the present invention is their small size. Typically, the cross
sectional area of the poles 16 (i.e., the terminal area inside of
the dotted lines shown in FIG. 2) is smaller than about 0.3 square
millimeter preferably smaller than about 0.2 square millimeter and
more preferably smaller than about 0.1 square millimeter.
[0040] A significant advantage of the QMSA of the present invention
is the extremely small size and dense spacing of the poles 16
forming the array. With continued reference to FIG. 2, in a
preferred embodiment, the face-to-face spacing (d1) between
adjacent, directly opposing poles 16 is smaller than about 0.2
millimeter, preferably smaller than about 0.15 millimeter, and most
preferably smaller than about 0.1 millimeter. Spacing (d2) between
diagonally opposing poles 16 is preferably smaller than about 0.3
millimeter, more preferably smaller than about 0.25 millimeter,
still more preferably smaller than about 0.2 millimeter and most
preferably smaller than about 0.15 millimeter. According to the
present invention, the density of quadrupoles in the QMSA is
typically greater than about 2 quadrupoles per square millimeter,
preferably greater than about 3 quadrupoles per square millimeter,
more preferably greater than about 4 quadrupoles per square
millimeter, and most preferably greater than about 5 quadrupoles
per square millimeter, with the area measured in a plane
perpendicular to the longitudinal axes of the quadrupoles in the
array. As used herein, a quadrupole refers to the equipotential
area, when the device is operating, in the area of a quadrupole
channel 17 defined by any grouping of four adjacent of the poles 16
of the array. With such a high density of quadrupoles per
cross-sectional area, the QMSA can easily accommodate 10
quadrupoles in devices designed for applications having even the
tightest space requirements, and more preferably at least 100
quadrupoles. The density of poles 16 in the array is preferably
greater than about 2 poles per square millimeter, more preferably
greater than 4 poles 16 per square millimeter, still more
preferably greater than about 6 poles 16 per square millimeter, and
most preferably greater than about 8 poles 16 per square
millimeter. Particularly preferred is a pole density in the array
of greater than about 10 poles 16 per square millimeter. With the
dense spacing of the adjacently located poles 16 and, thus, dense
spacing of quadrupoles, the spacing density of the quadrupole
channels 17 is typically one or more of the quadrupole channels 17
per square millimeter, and preferably more than about two of the
quadrupole channels 17 per square millimeter. When the array of the
poles 16 defines more than one quadrupole and, consequently more
than one of the quadrupole channels 17, the number of poles 16 will
be at least 6, and preferably at least 20 and more preferably at
least 100. Furthermore, the area of each of the quadrupole channels
17 for accepting ions (i.e., the area of the exemplified inscribed
circles in FIG. 2) is very small, typically smaller than about 0.05
square millimeter, preferably smaller than about 0.03 square
millimeter and more preferably smaller than about 0.02 square
millimeter.
[0041] The poles 16 of the array are positioned between the ion
source 28 and the ion detector 32 of the quadrupole mass
spectrometer such that substantially the entire length of each pole
16 is within the space between the ion source and the ion detector.
The poles 16 preferably have a length of shorter than about 6
millimeters, more preferably a length of shorter than about 4
millimeters, even more preferably a length of shorter than about 3
millimeters. In one embodiment, the length of the poles 16 is
shorter than about 2 millimeters.
[0042] The QMSA is part of the ion filter 29 of the present
invention. One embodiment of the ion filter 29 is shown in FIG. 3.
The ion filter 29 includes a thin patterned layer of electrically
conductive material, preferably of an electrically conductive metal
such as gold or titanium. The patterned layer includes a plurality
of elongated electrically conducting portions, each including in a
single integral piece a pole 16, a bonding pad 44 or 46, and a
connecting strip 50, with the connecting strip 50 being located
intermediate between the pole 16 and the bonding pad 44 or 46.
[0043] The pole 16 is located at one terminal end of each integral
piece, as previously described with reference to FIG. 2, and the
bonding pad 44 or 46 is located at the opposite terminal end. The
bonding pad 44 or 46 provides a location for making an electrical
connection to an external power source for providing power to the
array of the poles 16, and the connecting strip 50 provides an
electrical lead from the bonding pad 44 or 46 to the pole 16. As
shown, the bonding pad 44 or 46 has a greater width than the pole
16 or the connecting strip 50. Although not necessary to the
present invention, having a wider area available for bonding is
preferred for ease of making an electrical connection. Preferably,
the bonding pad 44 or 46 is suitable for making a wire bond
connection to an external power source.
[0044] Preferably, each of the integral pieces has a substantially
constant layer thickness (shown as dimension T in FIG. 3) for all
of the bonding pad 44 or 46, connecting strip 50 and pole 16.
Furthermore, it is preferred that all of the integral pieces making
up the patterned layer are of substantially the same thickness. A
substantially constant thickness for the patterned layer
facilitates ease of manufacture of the ion filter 29 and
incorporation of the ion filter 29 into a quadrupole mass
spectrometer. The thickness of the patterned layer is preferably
substantially equal to the length of the poles 16. The connecting
strips 50 preferably have a width (shown as dimension W in FIG. 3)
of smaller than about 0.5 millimeter.
[0045] The patterned layer of the ion filter 29 is typically
substantially all supported by a common substrate. This is
important both from a manufacturing perspective, as discussed
below, and from an operational perspective, due to the narrow
tolerances achievable when the integral pieces for all of the poles
16 are supported by a common substrate. The common substrate is
typically of a dielectric material. Examples of such dielectric
materials include alumina and glass. Furthermore, the common
substrate will typically include an opening over which the poles 16
and a portion of the connecting strips 50 are suspended. The
opening forms part of a pathway for ions traveling through the
device, as described more fully below. The ion filter 29 may be
supported on either side of the common substrate, the side facing
the ion source 28 or the side facing the ion detector 32.
[0046] The ion filter 29 of the present invention may be
incorporated into a quadrupole mass spectrometer in any convenient
way. One preferred configuration is shown in FIG. 4, which is an
exploded perspective view showing components of one embodiment of a
miniature micromachined quadrupole mass spectrometer 10. As shown
in FIG. 4, the quadrupole mass spectrometer 10 includes the ion
source 28, the ion filter 29 and the ion detector 32. The mass
spectrometer 10 also includes an entrance device 12, such as an
entrance plate, for controlling the movement of ions in the ion
beam 22 into the ion filter 29 and an exit device 14, such as an
exit plate, for controlling the movement of the mass filtered ions
30 from the ion filter 29. The mass spectrometer 10 also includes
an entrance spacer 18, and an exit spacer 20. During operation of
the mass spectrometer 10, the entrance device 12 receives ions in
the ion beam 22 from the ion source 28. Ions in the ion beam 22
pass through entrance apertures 24 extending through the entrance
device 12 to channel ions into quadrupole channels 17 (as shown in
FIG. 2) within the array of electrically conductive poles 16. The
exit device 14 is located at a distal end from the entrance device
12 and provides ions with egress through exit apertures 26
extending through the exit device 14. The mass-filtered ions 30
pass to the ion detector 32 for detection.
[0047] The array of poles 16 of the ion filter 29 is located
adjacent to and between the entrance device 12 and the exit device
14. The entrance spacer 18 maintains a predetermined spacing
between the array of poles 16 and the entrance device 12. The exit
spacer 20 maintains a predetermined spacing between the array of
poles 16 and the exit device 14. The exit spacer 20 also acts as a
common supporting substrate for the patterned layer of the ion
filter 29). One or both of the spacers 18, 20 may be bonded to the
structure of the ion filter 29 and to the entrance and exit devices
12, 14, respectively. As may be appreciated, many bonding methods,
preferably non-contaminating bonding methods, such as diffusion-
and amodic-bonding techniques, may be employed to obtain good
bonding results. The spacers 18, 20 may have any convenient
thickness, but typically each have a thickness of smaller than
about 1 millimeter and preferably smaller than about 0.5
millimeter.
[0048] Referring now to FIG. 5, a partial cross-section is shown
through a single opposing pair of the metallic poles 16 for the
mass spectrometer 10, except that the ion source 28 and the ion
detector 32 are not shown. As with the other figures, the
cross-section of FIG. 5 is not necessarily to scale and is shown
only for purposes of illustration.
[0049] Shown in FIG. 5 are the entrance device 12, including one of
the apertures 24, the exit device 14, including one of the
apertures 26, two directly opposing poles 16, the entrance spacer
18, and the exit spacer 20. Low dielectric-constant materials are
preferably used for the spacers 18, 20 to lower capacitance.
[0050] With reference to FIGS. 4 and 5, the poles 16 are preferably
non-magnetic, nonreactive, metallic rods, such as gold or titanium.
The spacers 18, 20 are insulators, preferably of glass, to isolate
the poles 16 during operation of the quadrupole mass spectrometer
10 of the present invention.
[0051] The entrance device 12 is important to at least partially
isolate the ion filter 29 and the ion source 28 and to channel ions
from the ion source into the ion filter 29. By acting as an
isolation shield, the entrance device 12 reduces the possibility of
detrimental interference between the ion source 28 and the ion
filter 29.
[0052] The exit device 14 is important to at least partially
isolate the ion filter 29 and the ion detector 32 and to channel
ions from the ion filter 29 to the ion detector 32. By acting as an
isolation shield, the exit device 12 reduces the possibility of
detrimental interference between the ion filter 29 and the ion
detector 32.
[0053] The entrance and exit devices 12, 14 may each be comprised
of substantially entirely only dielectric material. As shown in
FIG. 5, however, it is preferred that the entrance device 12 and
exit device 14 each include a dielectric interior body portion 34,
such as a silicon substrate 34, coated with an electrically
conductive outer layer 36, preferably a gold/chromium film layer
attached to and supported by the body portion 34. Preferably, the
electrically conductive outer layer 36 extends into the interior of
the apertures 24, 26, as shown in FIG. 5. The electrically
conductive outer layer 36 at least partially protects the array of
poles 16 during operation of the quadrupole mass spectrometer 10 by
dissipating the charge of ions that strike the outer layer 36. The
entrance device 12 may have a flat or concave surface for receiving
the ion beam 22, and the exit device 14 may have a flat or concave
surface for directing the exiting mass-filtered ions 30. As shown
in FIGS. 4 and 5, the surfaces are concave. Furthermore, although
it is most preferred that the electrically conductive outer layers
36 completely surround the entrance device 12 and exit device 14,
as shown in FIG. 5, such complete surrounding is not required.
Preferably, however, the conductive outer layer 36 of the entrance
device 12 covers at least a portion of, and more preferably
substantially all of, the surface of the entrance device 12 facing
the ion source 28. Likewise, it is preferred that the conductive
layer 32 of the exit device 14 cover at least a portion of, and
more preferably substantially all of, the surface of the exit
device 14 facing the ion filter 29.
[0054] The ion detector 32 is preferably any suitable detector for
detecting selected ions of the ion beam 22 in accordance with the
invention, such as a Faraday-type ion detector or a single-particle
multiplier detector.
[0055] With reference primarily to FIG. 4, the ion filter 29 is
shown, including the poles 16. The area 52 shown in FIG. 4 is that
portion of the ion filter 29 shown in larger scale in FIG. 2. The
connecting strips 50 radiate outward from the poles 16 and
terminate in electrical connection with one of either bonding pads
44 or bonding pads 46. One of the bonding pads (either 44 or 46),
the associated connecting strip 50 and the associated pole 16 are
typically manufactured as an integral unit, as described more fully
below with the discussion concerning preferred manufacturing
methods for making the ion filter 29. Also, the bonding pads 44 and
the bonding pads 46 are offset, so that electrical connections may
more easily be made to the bonding pads 44, 46. During operation of
the mass spectrometer 10, an RF frequency voltage and a DC voltage,
as described previously, are applied to the poles 16 via electrical
connections made to the bonding pads 44, 46. The specific frequency
and magnitude of the RF voltage and the specific magnitude of the
DC voltage applied to the poles 16 determine the value of m/z for
ions passing through the ion filter 29 to exit with the mass
filtered ions 30 for detection. By varying the frequency and/or
voltages, the selected m/z for ions passing through the ion filter
29 may be varied. By continuously or periodically varying the RF
frequency and voltages over a predetermined range, the mass
spectrometer 10 may be used to scan for ions over a wide range of
m/z values. The mass spectrometer 10 may be designed for m/z
detection in the range of m/z of from about 1 to about 4000. For
many applications, however, the range for m/z detection with the
mass spectrometer 10 is from an m/z of about 1 to an m/z of about
300.
[0056] With continued reference to FIG. 4, the patterned layer of
the ion filter is substantially entirely supported by the exit
spacer 20, which acts as a common supporting substrate. The exit
spacer 20 has an opening 35 through the exit spacer 20. As the ion
filter 29 is supported by the exit spacer 20, the opening 35 and
the ion filter 29 are aligned so that at least the area 52 of the
ion filter, including the poles 16 and portions of the connecting
strips 50, are positioned over the opening 35. Therefore, the poles
16 and at least a portion of the connecting strips 50 are suspended
from the exit spacer 20 over the opening 35. The opening 35 forms
part of a pathway permitting ions from the ion source 28 to travel
through the ion filter 29 to the ion detector 32. This pathway
includes an entrance aperture 24 through the entrance device 12, an
opening 37 through the entrance spacer 18, the quadrupole channels
17 (shown in FIG. 2) through the array of the poles 16, the opening
35 through the exit spacer 20 and the exit apertures 26 through the
exit device 14.
[0057] It will be recognized that the relationship between the
poles 16 and a common supporting substrate may involve different
geometries in the mass spectrometer 10 without departing from the
spirit of the invention. For example, the common supporting
substrate could include a plurality of openings, rather than just
one opening, with a different group of the poles 16 suspended over
each of the plurality of openings. Also, the common supporting
substrate could be used as an entrance spacer, rather than an exit
spacer, with the ion filter supported on the side facing away from
the ion source 29, rather than toward the ion source 29, as is
shown in FIGS. 4 and 5, and an exit spacer could thus be used that
is of similar design to the entrance spacer 18 as shown in FIGS. 4
and 5.
[0058] The mass spectrometer 10 may be operated at any convenient
RF frequency. Typically, however, the length of the poles 16 (shown
as the dimension L.sub.P in FIG. 5) will be short enough to permit
operation of the quadrupole mass spectrometer at low RF
frequencies, such as frequencies less than about 50 MHz, which is
generally preferred. This lower operational frequency allows the
voltages V.sub.0 and U.sub.0 to be maintained at conveniently low
values for the desired mass range to reduce the possibility of
arcing across closely-spaced parts and to minimize power
consumption in the electronics and radiation (varying as the sixth
power of frequency). For example, a convenient length, L.sub.P, of
the poles 16 may range from about 2 mm to about 6 mm, as previously
discussed, and may even be selected to be shorter than about 2
mm.
[0059] The use of short poles 16 and a Faraday-type ion detector
allows operation at higher pressures, often in the millitorr range,
wherein the particle's mean free path length may be comparable to
instrument dimensions. As will be appreciated, operation at higher
pressures allows the use of a smaller, less expensive backing pump
to create the required vacuum conditions, rather than using, for
example, a larger, higher-speed turbomolecular pump in combination
with a backing pump.
[0060] The entrance device 12, spacers 18 and 20, bonding pads 44
and 46, and exit device 14 may have electrically conductive
surfaces since they are located near charged-particle beams to
produce known and fixed particle energies. As will be appreciated,
the materials used to fabricate all the components preferably have
coefficients of thermal expansion that are low enough to control
distortion caused by operational temperature variations.
[0061] As noted previously, the poles 16 may have a hyperbolic
shape (to follow the original Mathieu-equation formulation of the
quadrupole problem). However, the poles 16 may also have other
shapes with negligible loss in mass resolution, such as cylindrical
(i.e., with a semicircle or other circle arc section at the
terminal ends forming the poles 16). Other shapes may provide
easier final fabrication of plating molds (discussed below) for the
poles 16 and, possibly, a denser packing of the poles 16.
[0062] During operation of the mass spectrometer 10, of a
configuration as shown in FIG. 4, portions of the incident ion beam
22 passes through the entrance apertures 24 contained within the
entrance device 12. Each of the entrance apertures 24 should
correspond to and be aligned with one of the quadrupole channels 17
(shown in FIG. 2) within the array of poles 16, so that the
entrance apertures 24 channel ions form the ion source 28 to the
ion filter 29. Ions from the ion beam 22 that pass through the
apertures 24 then travel through the array of the poles 16 of the
ion filter 29. Ions exiting the ion filter 29 then depart through
the exit apertures 26 contained within the exit device 14 as the
mass-filtered ions 30 to be detected by the ion detector 32. Each
of the exit apertures 26 should correspond to and be aligned with
one of the quadrupole channels 17 (shown in FIG. 2) within the
array of poles 16, so that the entrance apertures 24 channel ions
exiting the ion filter 29 to the ion detector 32.
[0063] Detection sensitivity lost in miniaturization may be at
least partially overcome by the use of numerous quadrupoles working
in parallel as shown in FIGS. 4 and 5. As will be appreciated,
miniaturization tends to reduce detection sensitivity because fewer
particles can be admitted into the reduced entrance apertures 24 of
the mass spectrometer 10. Thus, the basic pattern, described above
and shown in FIGS. 2-5, can be repeated 1 to 10,000 times or more
(depending on the desired results) to form a desired array of poles
16. Moreover, the poles 16 may be wired to all work in parallel, or
different parts of the array of the poles 16 can be tuned to
different mass ranges. As will be appreciated, variable control
over operations of the spectrometer 10 may be useful when
monitoring, for example, in an atmosphere or plasma, a transient
phenomena, or a spatially-variable phenomena.
[0064] Referring now primarily to FIGS. 4, 6 and 7, a preferred
manner for making electrical connections to the poles will now be
described. FIG. 6 illustrates a perspective view of one type of
bonding configuration and FIG. 7 shows a single quadrupole device
for illustrating bonding configurations and electrical connections.
The metal connecting strips 50 are attached between the bonding
pads 44, 46 and the poles 16 to support the poles 16 of the ion
filter 29 suspended over the opening 35 through the exit spacer 20
and to electrically connect the poles 16 to an RF generator (not
shown). The bonding pads 44, 46 are each at a terminal end of the
integral piece opposite the poles 16. The bonding pads 44, 46
provide additional structural strength for each connected pole 16
and for providing a site for wire bonding at the top of these
structures as a secondary method of electrical connectivity.
[0065] As shown in FIGS. 6 and 7, the array of the present
invention may have parallel wiring in an easy-access configuration.
For example, dual tracks, a Track A 40 and a Track B 42, may be
used with the dual bonding pads 44, 46 (one for each track) and the
metal connecting strips 50 to electrically connect the bonding pads
44, 46 with the poles 16. The metal connecting strips 50 are
connected to alternate positive (+) and negative (-) poles 16 of
the quadrupole array. Outer metal Track A 40 and inner Track B 42
provide parallel access to the positive (+) and negative (-) poles
16, respectively. For example, all the positive (+) poles 16 may be
connected to Track A 40, and all the negative (-) poles 16 may be
connected to Track B 42, or vice versa.
[0066] The dual bonding pads 44, 46, one for Track A 40 and one for
Track B 42, have a sufficient bonding surface, such as
approximately 1 mm by 3 mm. The bonding pad 44 of Track A 40 is
preferably at least approximately 0.5 mm from Track B 42 so that
there is sufficient clearance between Track A 40 and Track B 42.
Electrical connectivity is realized by wire bonding, pressure
contacting, or electroplating the structure from a
previously-patterned substrate, such as exit spacer 20 of FIG. 4.
The conducting poles 16, the connecting strips 50 and the bonding
pads 44, 46, along with the dual tracks 40, 42 form the ion filter
29 for this embodiment. The exit spacer 20 (as shown in FIG. 4)
preferably includes an electrically conductive bonding pattern 33,
which is a patterned electrically conductive film that has a
pattern that matches and corresponds with the pattern of the
connecting strips 50 and the bonding pads 44, 46. The bonding
pattern 33 enhances the ability to securely bond the ion filter 29
to the exit spacer 20. Furthermore, bonding of the connecting
strips 50 and bonding pads 44, 46 securely to the exit spacer 20
maintains the poles 16 with the desired orientation with the poles
suspended over the opening 35.
[0067] The present invention recognizes that several fabrication
methods may be employed to produce the ion filter 29 of the present
invention. It is important, however, that the manufacture method be
such that the poles 16, as manufactured, have alignment and
relative positioning for final use in a quadrupole mass
spectrometer. This is typically accomplished by forming the
patterned layer of the ion filter 29 so that it is all
substantially supported by a common supporting substrate, such as
the exit spacer 20.
[0068] One such method of the present invention for making the ion
filter 29 quadrupole array includes the simultaneous fabrication of
the poles 16, such as by simultaneously forming the poles 16, and
typically also simultaneously forming the remainder of the
patterned layer of the ion filter 29, in a mold by filling the
pattern of the mold with electrically conductive material. In a
preferred embodiment, the mold includes the pattern for all of the
poles 16, the connecting strips 50 and the bonding pads 44, 46,
which are all then fabricated simultaneously by filling the mold.
As may be appreciated, the mold may be produced in a separate
process or included as a step(s) in making the ion filter 29 of the
present invention. Although other methods may be acceptable, one
preferred means of creating the mold is through
Lithographie-Galvanoformu- ng-Abformung (LIGA) manufacture,
discussed in more detail below. Similarly, any acceptable method
may be used to fill the mold with electrically conductive material,
such as, for example, by electroplating, chemical vapor deposition,
physical vapor deposition, or loading voids in the mold with
nanoparticles of the desired material. LIGA manufacture is
particularly useful for poles 16 having lengths in a range of from
about 0.5 mm to about 6 mm, and preferably of from about 0.5 mm to
about 4 mm.
[0069] Another method of making the array of the poles 16 involves
precise selective removal of portions of a work piece, that is
initially a single solid sheet of electrically conductive material,
to obtain the desired patterned layer for the ion filter 29. It is
preferred that all of the poles 16, the connecting strips 50 and
the bonding pads 44, 46 be manufactured from the same work piece
and that the final patterning be done only when the single work
piece is supported by a common substrate, such as the exit spacer
20. The selective removal may be any suitable technique. In this
regard, Electrical Discharge Machining (EDM), discussed in detail
below, may be employed to selectively remove material from the work
piece and thereby obtain acceptable tolerances for poles 16. EDM
manufacture is particularly preferred for manufacturing poles
having a length of at least about 4 mm.
[0070] As will be appreciated, the use of the LIGA and EDM
fabrication methods facilitates the production of poles 16 of a
quadrupole array having the desired relative positioning of the
poles 16 in a high density array. In this regard, the density and
small size of the array is advantageously achieved by forming all
of the poles 16 so that, as manufactured, the patterned layer,
including the poles 16, the connecting strips 50 and the bonding
pads 44, 46, is supported by a single substrate (e.g., the exit
spacer 20). It should, however, be recognized that, although it is
preferred that the method of the invention may be used to fabricate
the entire patterned layer of an ion filter 29, the invention is
not so limited. The method could be used, for example, to
manufacture only an array of poles 16 in alignment, with electrical
connections to the poles 16 being made other than through the
connecting strips 50 and bonding pads 44, 46.
[0071] With EDM-based manufacture, all of the poles 16 and other
portions of the patterned layer of the ion filter 29 are formed by
selective removal of material from a single piece of electrically
conductive material that has been first bonded to and supported on
a common substrate (e.g., exit spacer 20). In the case of
LIGA-based manufacture, the poles 16 and portions of the patterned
layer of the ion filter 29 are formed in a single operation by
filling a mold, with the mold being located over a common
supporting substrate (e.g., exit spacer 20) so that the patterned
layer of the ion filter 29 will be supported by the common
supporting substrate. In this manner, proper alignment of the poles
16 is established concurrently with manufacture of the poles 16. By
manufacturing the poles 16 so that, as manufactured, they are
supported by a common supporting substrate, problems associated
with positioning and aligning preformed rods, as is encountered
with manufacture of conventional quadrupole devices, may be
avoided. Rather, with the present invention, positioning and
alignment of the poles 16 are accomplished during the same process
operation in which the poles 16 are formed, considerably
simplifying manufacture of the ion filter 29 by eliminating steps
involving positioning and aligning loose, preformed rods.
[0072] METHOD OF FABRICATION USING A MOLD
[0073] The manufacturing method of the present invention will now
be exemplified with a description of one embodiment of the method
involving formation of an array of poles, and other portions of the
patterned layer of the ion filter, by filling a mold. Preparation
of the mold by the LIGA technique is also described, although it
will be appreciated that the mold could be made by any suitable
technique or could be acquired from an external source, such as an
outside specialty manufacturer. FIG. 8 shows a process flow diagram
illustrating one embodiment of the LIGA-based fabrication process
of the present invention. It will be appreciated that the order of
the steps is intended to be only illustrative in nature.
[0074] The LIGA method is employed in the present invention to
manufacture a mold, which is also sometimes also referred to as a
template. The mold may be made of any suitable material, but is
typically a polymeric material, such as polymethyl methacrylate
(PMMA) or a polyimide. A preferred material for the mold is PMMA.
The discussion here will, therefore, be with reference to PMMA as
an example of the mold material. The same principles apply to other
mold materials. The molds are filled with an electrically
conductive material to form the patterned layer of the ion filter,
including an array of the poles. Because electroplating is a
preferred method for filling the molds, the process is discussed
with reference to electroplating by way of example. The same
principles apply, however, to other methods for filling the
mold.
[0075] To manufacture a quadrupole mass spectrometer with the ion
filter, other components such as entrance and exit devices and
spacers are manufactured and then modularly assembled with the ion
filter. The resulting quadrupole mass spectrometer is typically
{fraction (1/50)}th, or smaller, of the mass and volume of present
commercial quadrupole mass spectrometer devices. In that regard,
the quadrupole mass spectrometer 10, as shown in FIGS. 4 and 5, may
have a weight of smaller than about 7 grams and may occupy a total
volume of smaller than about 2 cubic centimeters. Detection
sensitivity lost in miniaturization may be at least partially
overcome by fabricating the ion filter with a plurality of
quadrupoles working in parallel, thereby increasing the area
available for ion travel. For example, the ion filter of the
present invention could include 10, 100 or even 10,000 or more
quadrupoles. Although it will be appreciated that as the number of
quadrupoles becomes very large, the size of the device will
necessarily increase.
[0076] Using LIGA-based techniques, fabrication of the patterned
layer of the ion filter is accomplished, for example, through
electron-beam lithography (to manufacture repetitive gold LIGA
X-ray masks using intermediate steps of contact-printing and
gold-plating) followed by X-ray exposure of the PMMA in a
synchrotron light source. The exposed PMMA is chemically developed
away, the pattern of void spaces are filled by electroplating with
electrically conductive material (gold or titanium is preferred),
and exit and entrance spacers and entrance and exit devices having
apertures are provided for assembly. After these components are
aligned, assembled, and bonded together, an RF generator may be
connected (e.g., through wire bonding techniques) and an ion source
and ion detector provided to complete fabrication of a mass
spectrometer.
[0077] LIGA-based processing is suitable to this manufacture
because it is capable of producing high dimensional accuracy which
allows the quadrupole array (e.g., poles) to be electroplated to a
close tolerance, preferably to within a 0.1% dimensional tolerance.
The LIGA method achieves this accuracy at least in part by using
computer-aided mask manufacture to create masks used in fabricating
the final template. To further improve the quality of the produced
quadrupole array, advanced bonding techniques, such as anodic,
diffusion, eutectic, or ultrasonic bonding, can be used to create
contamination-free, corrosion- and temperature-resistant bonds
without altering the dimensions of poles, connecting strips, and
bonding pads.
[0078] One Embodiment of LIGA-Based Fabrication:
[0079] With reference to FIG. 8 showing the sequence of processing
steps and FIG. 4 showing various components of the quadrupole mass
spectrometer 10, one embodiment of LIGA-based fabrication of the
patterned layer of the ion filter 29 is described.
[0080] (a) Fabricate Optical Mask:
[0081] In this step, an optical photomask is fabricated for
subsequent use in the fabrication of an X-ray mask. A standard
electron-beam lithography apparatus is used to etch the "footprint"
or pattern of the ion filter (i.e., poles 16, connecting strips 50,
and bonding pads 44, 46) in a resist material coating a quartz
substrate on which a UV opaque material, typically chromium, has
been previously deposited. In this regard, the electron beam can be
precisely controlled to an accuracy of about 1 nm in 1 cm. After
exposure to the electron beam, the undesired resist material is
developed away, and the entire mask is then placed in an etchant
bath to remove the chromium film from the exposed areas. The
remaining resist is then removed leaving the previously-protected
chromium pattern to be used as an optical mask for further
lithography.
[0082] (b) Fabricate X-Ray Mask:
[0083] The optical mask of step (a) is next used to fabricate an
X-ray mask (to be used in the subsequent exposures in the
synchrotron light source, see (c) below). The optical mask of step
(a) is laid over a plate consisting of a 50 micron layer of
photoresist coated over a 300 angstrom layer of gold, itself on a
50 angstrom layer of chromium, all supported on a silicon
substrate. The assembly is then exposed to collimated ultraviolet
(UV) radiation which replicates the pattern of (a) by passing
through the quartz-only portions of the optical mask. Next, the
undesired photoresist is developed away, and gold is then plated
into these developed regions. As can be appreciated, this process
creates a four-layer mask consisting of a patterned 50 micron gold
layer on a 300 angstrom gold layer, itself on a 50 angstrom
chromium layer, all on the silicon substrate.
[0084] (c) Expose PMMA Through X-Ray Mask:
[0085] A PMMA sheet, having a thickness slightly greater than the
final desired thickness of the patterned layer of the ion filter 29
is then exposed through the X-ray mask of step (b) to synchrotron
X-ray radiation. The excess thickness is provided to accommodate
lapping of the final structure, as discussed below. A synchrotron
light source is used because it provides a collimated, intense beam
of X-rays. These X-rays irradiate the PMMA sheet through the X-ray
mask at the thin-gold locations. Because the X-rays are blocked by
the thick-gold areas of the mask, the pattern of the ion filter is
replicated in the PMMA sheet. A single X-ray mask may be used to
pattern numerous PMMA sheets.
[0086] (d) Develop Exposed PMMA:
[0087] The PMMA sheet of step (c) is then placed in a suitable
mixture of solvents, such as methyl isobutyl ketone (MIBK), to
dissolve the portion of the PMMA sheet exposed to X-rays in step
(c). The solvent mixture is chosen so as not to dissolve or
otherwise deteriorate portions of the PMMA sheet not exposed to
X-rays. The resulting patterned PMMA sheet provides a template of
the ion filter that can now be used as a mold that can be filled
with electrically conductive material to form the patterned layer
of the ion filter 29, including the array of the poles 16 for the
quadrupole array of the present invention. The process up to this
point has been involved with making the mold. It should be
recognized, however, that the mold could be made by any suitable
technique or could be purchased in a premanufactured state from an
outside source.
[0088] (e) Fill PMMA Mold:
[0089] Using standard electroplating methods, the PMMA mold of step
(d) may now be filled with a selected electrically conductive
material (e.g., gold or titanium) to form the quadrupole array. To
facilitate electroplating and further fabrication of the quadrupole
mass spectrometer of the present invention, the PMMA mold may be
placed on a electrically conductive base on a common supporting
substrate (e.g., bonding pattern 33 on exit spacer 20) that will
form part of the finally assembled mass spectrometer. Because the
exit spacer 20 is preferably fabricated from a electrically
non-conductive material (e.g., ceramic or other dielectric), the
electrically conductive bonding pattern 33 is bonded to the exit
spacer 20 prior to placing the PMMA mold on the exit spacer 20,
typically by standard thin film or thick film deposition
techniques. It will be appreciated that at this point in the
manufacture process, the exit spacer 20 will not include the
opening 35, so that there will be a solid surface to electroplate
against in the area that the opening 35 will eventually occupy.
[0090] A typical way to provide the bonding pattern 33 on the exit
spacer 20 is to initially deposit a continuous film of electrically
conductive material (e.g., gold) on the surface of the exit spacer
20 (i.e., the ceramic material is metallized). The pattern of the
ion filter 29 is then lithographically imprinted in this
electrically conductive film, and the exit spacer 20, with the
lithographically imprinted film, is placed in an etchant bath to
selectively remove the electrically conductive film from the
exposed areas, thereby forming the electrically conductive bonding
pattern 33. In this manner, the bonding pattern 33 is produced on,
and bonded to, exit spacer 20. The PMMA mold is now located on the
exit spacer 20 so that the bonding pattern 33 is aligned with the
pattern for the ion filter 29 in the PMMA mold. The PMMA mold is
filled with the appropriate electrically conductive material (e.g.,
gold or titanium) by electroplating to the bonding pattern 33 that
is exposed through the PMMA mold. The final electroplated structure
is lapped (e.g., abrasive lapping with a fine-diameter slurry) to
provide a flat planar surface having a desired surface finish for
subsequent processing and to establish the desired final thickness
of the patterned layer of the ion filter 29, which is equal to the
desired final length of the poles 16.
[0091] (f) Dissolve PMMA Mold:
[0092] After the filled PMMA mold has been lapped, the remaining
PMMA of the mold is then dissolved in a solvent, such as methylene
chloride, leaving a free-standing structure of the ion filter 29
(including the array of poles 16, the connecting strips 50 and the
bonding pads 44, 46) bonded to the corresponding bonding pattern 33
and supported by the exit spacer 20. Also, as will be appreciated,
the mold may be removed by techniques other than dissolution in a
solvent. For example, the material of the mold could be removed by
laser ablation. The exit spacer 20 may be machined to create the
opening 35 before or after the mold is removed. As will be
appreciated, the opening 35 may be produced by employing various
machining methods. A preferred technique is ultrasonic machining.
For example, ultrasonic impact drilling may be used which involves
placing an abrasive slurry in contact with exit spacer 20 and then
using a tool, having the shape of the desired opening 35, to
rapidly (e.g., reciprocating vibrations at 15 to 30 kHz or higher)
and forcefully agitate the fine abrasive materials in the slurry,
thereby removing material of the exit spacer 20 to form the opening
35.
[0093] The ion filter 29 may now be assembled with other components
to make the quadrupole mass spectrometer 10. For example, the
entrance spacer 18, typically of glass, may be placed on the
exposed-and-lapped surface of the ion filter 29, and the entrance
device 12 then placed above the entrance spacer 18. The exit device
14 may then be bonded or clamped to the underside of the exit
spacer 20. As will be appreciated, alignment of these components
may be facilitated through the use of fiducial marks. The entire
assembly may then be bonded in place using methods including, for
example, the use of adhesives (of low vapor pressure, so as not to
cause contamination), anodic bonding, thermal compression bonding,
diffusion bonding, glass-to-metal seals, gold eutectic solder, or
constraining the assembly in place through non-deforming mechanical
clamping. The ion source 28 may then be coupled to the entrance
device 12, and the ion detector 32 connected to the exit device 14,
and an RF generator may be connected to the bonding pads 44, 46 to
make the device functional.
[0094] It should be recognized that in the broadest sense, the
manufacture method of the present invention involving the use of a
mold to form the pattern of the poles 16 need not include all of
the steps described with reference to FIG. 8. Rather, it is
sufficient that a mold be used to form the pattern so that the
poles 16, as they are formed in the mold, have relative positioning
and alignment for use in a quadrupole mass spectrometer.
[0095] METHOD OF FABRICATION USING EDM TECHNIQUES
[0096] FIG. 9 shows a process flow diagram illustrating one
embodiment of the Electrical Discharge Machining (EDM) based
process of the present invention. EDM is a machining process that
selectively removes metallic material from a work piece by spark
erosion. Unlike conventional machining, which mechanically shears
tiny strips from the workpiece, EDM uses alternating current (AC)
or direct current (DC) from a special generator to melt and
vaporize conductive material away from the workpiece. Cooling and
cleaning is usually provided by pumping deionized water through the
cutting region. In a preferred embodiment, the present invention
includes a small diameter (e.g., 0.001 inch) alloy wire electrode
that is driven by machines with accurate computer-controlled drives
in the x, y and z axes. The machines are computer programmed to
give the desired final geometry and dimensions of the
workpiece.
[0097] One Embodiment of EDM-Based Fabrication:
[0098] With reference to FIG. 9 showing the sequence of processing
steps and to FIG. 4 showing various components of the quadrupole
mass spectrometer 10, one embodiment of EDM fabrication of the
patterned layer of the ion filter 29 is described.
[0099] (a) Bond Work Piece to Substrate:
[0100] A supporting substrate (e.g., exit spacer 20) is provided
having the bonding pattern 33. To the bonding pattern 33 is bonded
a single work piece, in the form of a sheet of electrically
conductive metal (e.g., gold or titanium). The sheet preferably has
a thickness that is substantially equal to the desired thickness
for the final patterned layer of the ion filter 29, and therefore
also substantially equal to the desired final length of the poles
16. The bonding pattern 33 may have been formed on the exit spacer
20 as previously described in the discussion concerning LIGA-based
manufacture. Bonding of the work piece to the bonding pattern 33 on
the substrate may be accomplished in any suitable manner. A
preferred manner of bonding is by the use of solder placed between
the bonding pattern 33 and the work piece. Also, it is preferred
that at the time the work piece is bonded to the exit spacer 20,
the exit spacer already has the opening 35 therethough. It is,
however, possible to make the opening 35 after the work piece has
been bonded to the exit spacer 20, if desired. Also, the opening 35
may be made before or after the bonding pattern 33 has been formed
on the exit spacer 20.
[0101] (b) Pattern Work Piece:
[0102] After the work piece has been bonded to the substrate, wire
EDM is used to selectively remove material from the work piece to
form the patterned layer of the ion filter 29, including the poles
16, connecting strips 50 and bonding pads 44 and 46. The geometry
and accuracy of the selections removed are controlled by the
software and accurate x, y, and z directional drives and is
preferably to within a 0.1% dimensional tolerance. As will be
appreciated, the metallic work piece may have been at least
partially patterned (through EDM or other methods) prior to being
bonded in step (a) to the bonding surface on exit spacer 20. For
example, the bonding pads 44 and 46 and the connecting strips 50
may be at least partially patterned prior to bonding to the exit
spacer 20, simplifying the patterning of the work piece on the
substrate. It is important, however, that the final division of the
work piece into the separate integral pieces for each of the poles
16 not occur until after the work piece has been bonded to the exit
spacer 20. In this way, the poles 16 are formed with the proper
positioning and alignment for use in a quadrupole mass
spectrometer, with the positioning and alignment being retained by
the bond to the exit spacer 20.
[0103] It should be appreciated that in its broadest sense, the EDM
processing of the present invention does not require the first step
shown in FIG. 9, i.e., the bonding step. The substrate could be
acquired from an outside supplier with the work piece already
bonded to the substrate. It is sufficient that selective removal of
material from the work piece bonded to the substrate occur in a
manner such that the poles 16, as they are formed, have the
relative positioning and alignment for use in a quadrupole mass
spectrometer.
[0104] After the work piece has been patterned into the patterned
layer of the ion filter 29, then the ion filter 29 may be
assembled, along with other components, into the mass spectrometer
10, in a manner as previously described.
[0105] Applications
[0106] As will be appreciated, the use of the above discussed
LIGA-based and EDM-based fabrication processes facilitate the
production of accurate, miniature quadrupole mass spectrometers
with reduced complexity of manufacture relative to conventional
manufacture of quadrupole mass spectrometers. It is anticipated
that the reduced cost and advantageous size of the quadrupole mass
spectrometer of the present invention will have many commercial
applications. In this regard, the miniature quadrupole mass
spectrometer of the present invention may be used for process
control, personnel safety, and pollution monitoring. Also, the
small size of the present invention allows small sensors containing
the miniature quadrupole mass spectrometer to be manufactured.
Commercial applications of the small sensors may include
distributing the sensors throughout manufacturing plants., in
public areas (such as buildings and subway systems), within plasma
chambers (chip manufacturers), in earth-orbiting space stations, in
long-duration human flight missions, for planetary aeronomy and
planetary-surface studies, etc. Other commercial applications of
the present invention may include automotive exhaust monitoring,
home fire/radon/CO monitoring, personnel environmental monitoring,
smokestack monitoring, and down-hole monitoring.
[0107] Also, because of the small size of the device, a high vacuum
may not be required in some applications. This is because the
requirement of small particle mean free path relative to the
(small) spacing of the poles, as described above, can now be met
with the present invention at a higher ambient pressure. This
obviates the need for sophisticated pumping and can place devices
of the present invention into the realm of operation of, for
example, micromachined peristaltic pumps. Use at the higher
pressures would require a pressure-resistant electron emitter (such
as a field ionizer) to ionize the neutral species and a Faraday cup
as the ion detector.
[0108] Furthermore, although the present invention has been
described primarily in reference to the quadrupole mass
spectrometer, the invention, in its broadest aspects is not so
limited. Rather, one important aspect of the present invention
relates to the ion filter described herein and methods for making
the ion filter.
[0109] Moreover, while the invention has been described in
combination with specific embodiments thereof, it is evident that
many alternatives, modifications, and variations will be apparent
to those skilled in the art in light of the foregoing description.
Specifically, it should be understood that the order of the
fabrication and assembly of the present invention may be altered
from that given as an illustration. Further, it should be
understood that a fabrication step may be omitted (e.g., by
purchasing a prefabricated component) and still be within the
spirit of the present invention. Accordingly, it is intended to
embrace all such alternatives, modifications, and variations as
fall within the spirit and broad scope of the appended claims.
* * * * *